All solids have surface roughness which has a huge influence on a large number of physical
phenomena such as adhesion, friction, contact mechanics and the leakage of seals[1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}.
Thus when two solids with nominally flat surfaces are squeezed into contact, unless the applied
squeezing pressure is high enough, or the elastic modulus of at least one of the solids low enough,
a non-contact region will occur at the interface. If the non-contact region percolate
open flow channels exist, extending from one side of the nominal contact region to the other side.
This will allow fluid to flow at the interface from a high fluid pressure
region to a low pressure region.
